The present invention relates to a method and to a corresponding measuring machine for determining dimensional properties of a measurement object. In particular, the invention relates to a method for measuring a measurement object using what is known as a profile projector, i.e. using a measuring machine that provides a 2D image of the measurement object on a display in order to measure, for example, the position and/or the profile of an object edge.
DE 198 05 040 A1 discloses a prior art measuring machine having a workpiece table for supporting a measurement object and having a camera that is arranged above the workpiece table and is displaceable vertically. Arranged below the workpiece table is a light source that illuminates the measurement object in what is known as transmitted light. The image recorded by the camera consequently shows the measurement object as a silhouette. However, more specific details of the measurement method are not described therein.
One example of a digital profile projector is disclosed by US 2010/0225666 A1. This profile projector generates a digital image of a measurement object, and a drawing of the measurement object is superimposed onto the image on a display so as to be able to check whether the measurement object corresponds to the drawing within predetermined workpiece tolerances.
The imaging objects of a real optical system and, consequently, also the camera of a real optical measuring machine are known to always exhibit aberrations that can be traced back in part to manufacturing tolerances and in part to trade-offs during the optical design and/or to underlying physical phenomena. The aberrations represent the deviation of the real imaging optics from an ideal optical image that is possible only in theory. Typical aberrations include spherical aberration, astigmatism, coma, image field curvature (other term: focal plane deviation—FPD) and distortion. For increasing the measurement accuracy of an optical measuring machine, the aberrations in the image recorded by the imaging optics can be corrected by way of computation before, during or after the actual image evaluation, wherein calibration values obtained in a previous calibration operation are used. U.S. Pat. No. 6,538,691 B1, for example, describes a computer-implemented correction of image distortions of a digital camera.
The known methods and measuring machines and the calibration methods used therein are not yet optimal, especially in cases where the imaging optics can be adjusted to different working distances relative to a workpiece table. The aberrations are frequently minimized in optimum fashion only for a specific working distance.